I don't think you can blame the Duke for how the maids acted. He assumed that they would treat her appropriately. That is their job.
People seem to be objecting to the Duke saying that he'll give her "as luxurious life". That it's not good enough for the MC. However, that is exactly what the MC wants. She wants to live in the mansion quietly, not bothering the Duke and the "heroine", until he's ready the marry the heroine. And as far as I can tell, that is exactly what the Duke wants.
It's quite amusing though. Her father insisted that the MC can annul the marriage whenever she wants. Although she isn't aware of it, it is something of a Damocles sword over the Duke's head.
Essentially the situation is where the MC doesn't need to care what the Duke thinks, beyond not getting in his and the heroine's way. But the Duke needs to avoid her annulling the marriage. At least in the short term. If she decides to annul the marriage now it will probably be really difficult to get another fiance.